Acoustic telemetry reveals cryptic residency of whale sharks

Keywords

Whale shark

Abstract

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​Although whale sharks (Rhincodon typus) have been documented to
move thousands of kilometres, they are most frequently observed at a
few predictable seasonal aggregation sites. The absence of sharks at the
surface during visual surveys has led to the assumption that sharks
disperse to places unknown during the long ‘off-seasons’ at most of
these locations. Here we compare 2 years of R. typus visual
sighting records from Mafia Island in Tanzania to concurrent acoustic
telemetry of tagged individuals. Sightings revealed a clear seasonal
pattern with a peak between October and February and no sharks observed
at other times. By contrast, acoustic telemetry demonstrated year-round
residency of R. typus. The sharks use a different habitat in
the off-season, swimming deeper and further away from shore, presumably
in response to prey distributions. This behavioural change reduces the
sharks' visibility, giving the false impression that they have left the
area. We demonstrate, for the first time to our knowledge, year-round
residency of unprovisioned, individual R. typus at an
aggregation site, and highlight the importance of using multiple
techniques to study the movement ecology of marine megafauna.